Long Term Objective: To define the role of ERK in pituitary development Aim 1. What is the requirement for ERK signaling during pituitary organogenesis? The working hypothesis underlying Aim 1 is that ERK signaling is necessary for anterior pituitary proliferation and/or cell lineage commitments. Aim 1 seeks to examine the in vivo role of ERKs on pituitary organogenesis using two separate models for Cre recombinase-mediated ERK gene deletion. We will focus on comparing an early global ERK knockout using the Pitxl Cre-expressing mouse line with an aGSU-CRE model where ERK deletion is lineage restricted to establish the role of ERK signaling during pituitary development. These studies include an analysis of histopathology, cell proliferation, survival and a complete assessment of lineage specification using immunohistochemistry for markers of lineage identity during pituitary development. These studies will, for the first time, define the requirement(s) for ERK signaling necessary for pituitary development and endocrine cell lineage commitment during pituitary organogenesis and set the framework for Aim 2 examining potential mechanisms of ERK signaling throughout the developing pituitary. Aim 2. What is the mechanistic relationship between onset of FGF signaling, activation state of ERKs, and downstream transcriptional responses during pituitary development? The hypothesis underlying Aim 2 is that ERK is activated in response to FGF signaling within the developing pituitary. Our preliminary studies provide compelling evidence that ERK activities are spatially and temporally regulated during development potentially correlated with FGF signaling gradients during Rathke's pouch formation. To directly test this, Aim 2 seeks to clarify the activity state of ERKs during organogenesis in an FGF KO mouse model. Next, we will establish the relationship between ERK and the well characterized downstream transcriptional networks that are crucial for pituitary development in our ERK KO mouse models. Initially, we will focus on immunohistochemical analysis the LIM homeodomain transcription factors, Lhx3 and lsl-1. Defining the role of ERK in pituitary development will lead to a broader understanding of the mechanisms involved in various dysfunctional pituitary conditions such as, CPHD and infertility.